The invention relates to a probe head for nuclear magnetic resonance measurements comprising a pick-up coil cooperating with a sample under investigation. The pick-up coil is connected to a first input and to a second input. The first input is used for feeding a signal of a higher radio frequency (f.sub.H) to the pick-up coil for exciting a first kind of nuclei, for example protons (H) and/or for receiving from the pick-up coil a resonance signal emitted by the nuclei of the first kind of nuclei (H). The second input is used for feeding a signal of a lower radio frequency (f.sub.X) to the pick-up coil for exciting a second kind of nuclei (X) and/or for receiving from the pick-up coil a resonance signal emitted by the nuclei of the second kind of nuclei (X). A radio frequency line is also connected to the pick-up coil. The radio frequency line has an electrical length corresponding to an integer multiple of a quarter wave length (.lambda./4) of the higher radio frequency (f.sub.H).
A probe head of the afore-mentioned kind is disclosed in U.S. Pat. No. 5,229,724 (Zeiger) assigned to the applicant of the present application.
Probe heads of the kind of interest in the present context are used for conducting nuclear double resonance experiments. In such experiments a first kind of nuclei, mostly protons (.sup.1 H) or fluorine (.sup.19 F) is excited and/or observed, whereas a second kind of nuclei is simultaneously excited and/or observed, for example certain isotopes of nitrogen (.sup.15 N) or phosphor (.sup.31 P) or carbon (.sup.13 C) or silicon (.sup.29 Si) or aluminum (27Al). In the art of magnetic resonance the first kind of nuclei is usually designated as "H" whereas the second kind of nuclei is identified as "X".
In modern high field nuclear magnetic resonance spectrometers, the exciting frequency for protons (.sup.1 H) is, for example, 800 MHz. In that case, the field strength of the constant magnetic field is about 18.8 Tesla. As is well-known in the art, the resonance frequency of nuclei and the magnetic field strength of the constant magnetic field are interrelated by a proportionality factor called the gyromagnetic ratio having an individual value for each kind of nuclei.
In the case specified before, i.e. always related to the same field strength of the magnetic field of 18.8 Tesla, the resonance frequency for the above-mentioned isotopes of nitrogen (.sup.15 N) is 81 MHz, of phosphor (.sup.31 P) is 324 MHz, of carbon (.sup.13 C) is 201 MHz, of silicon (.sup.29 Si) is 159 MHz and of aluminum (.sup.27 Al) is 208 MHz roughly.
The probe head according to U.S. Pat. No. 5,229,724 mentioned above is designed for a proton resonance frequency of 400 MHz. With a proton resonance frequency of 400 MHz, the resonance frequency of e.g. nitrogen (.sup.15 N) is only about 40.5 MHz. Therefore, the prior art probe head is designed for a frequency range of between 40 and 400 MHz.
The above-mentioned double resonance experiments are preferably conducted such that measurements are taken at X-frequency and decoupling is effected at H-frequency.
Probe heads for such experiments are designed such that the electric probe head network comprising the pick-up coil as well as the radio frequency line are optimized in their equivalent electrical circuit for the higher radio frequency, as seen from the first input and for the lower radio frequency, respectively, as viewed from the second input.
In the prior art probe head, a very broad frequency band may be swept on the X-side, i.e. on the lower radio frequency side, in contrast to still older prior art probe heads which had only been optimized for a very narrow frequency band on the X-side. In the probe head mentioned at the outset, one has attempted an optimization of between 40 MHz for .sup.15 N up to 162 MHz for .sup.31 P. The X-frequencies, therefore, amount to between one tenth and one half of the H-frequency being 400 MHz.
In the prior art probe head, the radio frequency line coupled to the pick-up coil is configured as a .lambda./2 line (related to the H-frequency of e.g. 400 MHz). At about one half of the .lambda./2 line, there is a switchable bridge so that the radio frequency line may be operated with its entire length (.lambda./2) when the bridge is open and at half length (.lambda./4) when the bridge is closed.
In the first case of the .lambda./2 line one has, therefore, an equivalent electrical circuit as viewed from the X-side in which the pick-up coil is terminated by a capacitance at its terminal end facing away from the X input (the second input). This results in a high pass characteristic in which the pick-up coil has a low resistive load for current of high frequency whereas, corresponding to the size of the capacitor, a lower threshold frequency has to be taken into account.
If, however, the bridge is closed and in the electrical equivalent circuit (again viewed from the X-side), the pick-up coil is terminated at its output by an inductivity so that in this situation the pick-up coil has a low resistive load for current of low frequency and, depending on the size of the inductivity, an upper threshold frequency may be determined.
Moreover, with the prior art probe head the electrical bridge must remain open for high X-frequencies (e.g. .sup.31 P) whereas it is closed for lower X-frequencies (e.g. .sup.15 N).
A disadvantage of the prior art probe head, when operated as a .lambda./4 line (as well as with other prior art probe heads utilizing a .lambda./4 line) the circuitry of the pick-up coil is asymmetric because the value of the radio frequency voltage at one terminal end of the pick-up coil is very high, whereas it is very low at the opposite end of the pick-up coil. This is highly disadvantageous in particular for high measuring frequencies.
Still another disadvantage of the prior art probe head, again when operated as a .lambda./4 line, is that the filling factor of the entire inductivity is not optimized because only a portion of the resonant circuit inductivity is filled with sample volume. This portion of the inductivity of the resonant circuit configured by the .lambda./4 line will be larger, the higher the X-frequency is. For that reason measurements at high X-frequencies are difficult so that supplemental tricks must be used, for example another inductivity switched in parallel to the pick-up coil.
Further probe heads of the kind of interest in the present context are disclosed in an article by Jiang, "An Efficient Double-Tuned .sup.13 C/.sup.1 H Probe Circuit for CP/MAS NMR and Its Importance in Linewidths", JOURNAL OF MAGNETIC RESONANCE, 71, 1987, pages 485-494, in U.S. Pat. No. 4,633,181 and in published PCT patent application WO 92/17792.
It is, therefore, an object underlying the present invention to improve a probe head of the kind mentioned at the outset such that improved measurements may be made at X-frequencies having up to one half the value of the H-frequency. Still another object is to enable improved measurements on very lossy samples which normally negatively affect the resonance frequency and the quality of the resonant circuit.